I had some private exchanges about this. It's a complex problem that comes up only with multiple nested frames and divs - and indeed, you can get a kind of "elastic" bounce-back of the scrollbar in Firefox.

It's not a bug I've seen reported, and indeed, the layout here was so complex I wonder if others will stumble over it. It takes a LOT of code to make it happen. IN fact the weird thing is that the error does not always render that way. In fact the page ends up with a different number of nested scrollbars on different browsers and even different screen sizes with the same browser.